An airbag system and a seatbelt pre-tensioner, which are occupant protecting systems, have been widely adopted in recent years for improving safety of the occupants in an automobile. The principle of the airbag system operation is that a gas generator is operated under control of signals from a sensor detecting a collision so as to inflate an airbag between occupants and a car body. The principle of the seatbelt pre-tensioner operation is substantially same, that a gas generator is operated under control of signals from a sensor detecting a collision so as to put occupants under constraint with seatbelts for protection. It is required of the gas generator to have a function for producing clean gas containing no harmful gas with a required and sufficient amount in a short time. And also it is required of the gas generator to be small in size and light in weight.
The gas generating agents for use in the gas generator are formed into a pellet form or a disc-like form by extrusion or pressure molding for stabilization of the burning. And it is required of the gas generating agents to maintain their initial combustion behavior over a long time even under various harsh environments. When the pellets deform or decrease in strength due to deterioration with age or change of environments and the like, the flammability of the explosive compositions will vary from that of the initial design then exhibit an abnormal combustion behavior. As a result of this, there is a possibility that the airbag or the gas generator may be broken itself in the event of the car-crash. In this case, it is failed to accomplish the aim of protecting the occupants. And there is even a possibility to cause them injury.
Gas generating agents containing metal azides such as sodium azide and potassium azide as their major component have been used as gas generating agents satisfying those required functions.
These known gas generating agents are widely used in terms of their various advantages such thet they are burnt immediately, the component of combustion gas is substantially nitrogen gas only, harmful gas such as CO (carbon monoxide) or NOx (nitrogen oxide) is not produced and it is easy to design the gas generator since the burning velocity is little influenced by the environment or the structure of the gas generator.
However, the metal azides have notable problems such that the metal azides itself are a harmful material, it produces azide easy to explode by impact and friction due to its contact with the heavy metal and it has decompose under the presence of water and acid then produce harmful gas. Thus, the metal azides must be handled with the greatest possible caution.
As the substitution of metal azides, gas generating agents containing tetrazoles, azodicarbonamides and other nitrogenous organic compounds as fuel components are proposed by, for example, Japanese Laid-open Patent Publications No. Hei 2(1990)-225159, No. Hei 2(1990)-225389, No. Hei 5(1993)-213687, No. Hei 6(1994)-32689 and No. Hei 6(1994)-80492, No. Hei 6(1994)-239684, No. Hei 7(1995)-206569 and No. Hei 7(1995)-206570.
The tetrazoles in particular are thermally stable and have a high proportion of atoms of nitrogen in their molecular structure, and thus have the property of inherently suppressing the production of CO. However, these involve the problem of readily producing NOx. So then, Japanese Laid-open Patent Publications No. Hei 2(1990)-225159 and No. Hei 3(1991)-208878 propose a method in which the gas generator is provided with a venturi means for introducing air into the combustion gas from outside so as to reduce the concentration of NOx as a whole. However, this method failed to clear up this problem essentially.
When nitrogenous organic compound is used as fuel, nitrate such as alkaline metal or alkaline earth metal, perchlorate or chlorate is generally used as an oxidizing agent for burning the nitrogenous organic compound. The alkaline metal or the alkaline earth metal contained in the oxidizing agent produces slag in the form of oxide or chloride as a result of the burning reaction. The proportion of the slag to the combustion products is not a little.
The occupant protection device may not serve since the oxide and chloride are harmful material for a human body and environment, and the oxide may cause damage to air bags to flow out of the gas generator. Accordingly, the oxide and chloride must be converted into slag in an easily collectable form, then the slag must collected in the gas generator. However, many of the gas generating agents using the nitrogenous organic compound as fuel have the calorific value as high as 2,000-3,000 joule/g or more. So, temperature and pressure of generated gas is high. Also temperature and flowability of slag is high, which is a by-product made in the burning of the gas generating agents. As a result of these, the slag collection efficiency of a filter fitted in a conventional type of gas generator tends to reduce. In order to increase slag collection efficiency, a method may be conceivable, wherein the slag is cooled and solidified by increased number of filtering members set in the gas generator. But such a method has a disadvantage of increasing the size of the gas generator, going against the trend toward the size reduction and weight reduction of the gas generator.
Japanese Laid-open Patent Publication No. Hei 4(1992)-265292 discloses another method for collecting the oxide of alkaline metal or alkaline earth metal which is produced in the reaction for burning the nitrogenous organic compound, wherein the oxide is converted into slag in the filtering part, the slag has a form easily collected, the slag is efficiently collected. According to this method, silicon dioxide or aluminum oxide is added as an acid or neutral slag forming agent that easily causes a slag-forming reaction with the oxides of alkaline metal or alkaline earth metal which are basic substances. However, those compounds do not in any manner contribute to the production of gas in the combustion reaction, thus resulting in reduction of the rate of gasification. Accordingly, the inventors have studied on how to improve the rate of gasification (a quantity of generated gas per unit weight of the gas generating agent) by using an oxidizing agent that produces no solid slag or a possible smallest quantity of solid slag after the combustion reaction, if any, as the oxidizing agent for the composition of the gas generating agent.
Ammonium nitrate and ammonium perchlorate can be cited as the oxidizing agent that produces no solid slag after combustion. One of the disadvantages that may arise from the use of ammonium nitrate as the oxidizing agent is that that substance causes various crystalline phase changes, among which the crystalline phase change that is caused at about 32° C. involves a great volume change. When the gas generating agent is repeatedly exposed to the crystalline phase changes above and below of that temperature range, the crystals of the ammonium nitrate expand and contract repeatedly to cause reduction in strength of or decay of the press-formed tablets of the gas generating agent which may become a possible cause of an abnormal combustion. For avoidance of this problem, a phase stabilizing method for ammonium nitrate is disclosed by PCT WO95/04710.
The ammonium nitrate is very low in reactivity, so a hazardous fuel component such as triaminoguanidine nitrate must be used to compensate for its hard burning property. Thus, the use of the ammonium nitrate as the oxidizing agent involves unavoidable problems of improvements in heat resistance and flammability.
On the other hand, the gas generating agents using the ammonium perchlorate are disclosed by Japanese Laid-open Patent Publications No. Hei 2(1990)-293389, No. Hei 5(1993)-221770 and No. Hei 8(1996)-228288. These are all produced by making use of the technology of propellant and are characterized by the use of a binder doubling as fuel. The binders doubling as the fuel that may be used include organic polymeric material, such as terminated hydroxyl polybutadiene, and silicon resin. The use of the organic polymeric material as the fuel constituent involves the inherent problem of increase in CO concentration in generated gas or aged deterioration resulting from the lack of heat resistance. Japanese Laid-open Patent Publications No. Hei 2(1990)-225159 and Hei 3(1991)-208878 disclose examples using nitrogenous organic compound as the fuel component and the ammonium perchlorate as the oxidizing agent. However, the composition of the gas generating agents could not be used singly for the protection of automobile occupants because of the poorness of the generated gas after combustion. Nevertheless, ammonium perchlorate is an interesting oxidizing agent in terms of heat resistance and reactability as the oxidizing agent, as compared with ammonium nitrate.
On the other hand, as a substitute for conventional stainless steel (SUS), aluminum is being widely used as a container material of the gas generator, for the purpose of weight saving of the gas generator. In the case of the container made of SUS, because of its excellent strength in high temperature, even when a temperature rise is caused by car fire, incineration of the gas generator or the like, no fracture of the container is caused and the composition of the gunpowder can be burnt out. In the case of the container made of aluminum, since its strength reduces significantly in high temperature, when the gas generator is exposed to flame of the car fire and the like and the composition of the gunpowder loaded in the interior is burnt, it is feared that the container cannot withstand the burning pressure and thus may be broken so that the fragments may be flied off to the surrounding to kill and wound occupants and persons around them. Accordingly, it is cited as the required term for the gas generator that the critical condition of the container, such as the fracture of the container, can be prevented even in such circumstances. To take measures to meet that situation, U.S. Pat. No. 4,561,675 proposed a system for the aluminum container, according to which the gunpowder that ignites automatically at a temperature lower than the temperature at which reduction of strength of aluminum is caused is arranged in close contact with an inner surface of the container. The automatic igniting gunpowder used therein includes nitrocellulose as a major component. Nitrocellulose itself lacks long-term stabilization under high temperature and further may ignite automatically due to that deterioration.
Smokeless powder having nitrocellulose as a major component has been equally used for the gas generating agent for use in the gas generator for a seatbelt pre-tensioner in terms of high burning velocity and autoignition capability, despite of the problems as mentioned above. Development of nitrocellulose is not originally intended for use in the gas generator and the oxygen balance in the composition (over and short oxygen in the combustion reaction) is not adjusted. Due to this, the use of nitrocellulose involves the problems of poorness in the combustion gas and very high combustion temperature.
The present invention aims to provide a gas generating agent that is good in generated gas composition and high in gasification rate by making choice of an oxidizer component of the gas generating agent having nitrogenous organic compound, nitroguanidine or aminotetrazole, in particular, as a fuel component which is a material effective for solving the problem of harmfulness of the metallic compound azide that has been used hitherto, whereby reduction in size and weight of the gas generator for use in the occupant protection device is accomplished.
Further preferably, the present invention aims provide the gas generating composition that is high in gasification rate, low in quantity of harmful NOx and CO gas components, excellent in heat resistance and small in volume of outflow slag and also holds an autoignition capability in the gas generating agent itself.